Adding Kafka Source to mupd8

Kafka is a real-time event messaging system. This change adds kafka source to mupd8. It enables user to consume from kafka directly, with few basic configurations. A kafka source requires three main parameters. 

1. Zookeeper connect string to connect to kafka
2. Consumer group name
3. Topic to consume from kafka
4. [Optional] key if message is json (similar to JSONSource)